As semiconductor integrated circuits continue to increase their packing densities by containing more devices with ever smaller dimensions, the problems involved in their fabrication, including electrically contacting individual devices, become more severe.
Electrical contacts are typically made by patterning a dielectric to form windows which expose selected portions of the substrate. The windows are filled with a metal to form the contact to the underlying substrate. Aluminum, the metal most commonly used in integrated circuits, is not easily used to fill small windows because of its poor step coverage. Alternatively, a blanket deposition of another metal, e.g., tungsten, which does have good step coverage may be used. The excess metal present on the dielectric surface is removed by etching to leave, in theory, a planar surface formed by the dielectric and the metal.
However, in practice, problems arise because the thickness of the deposited tungsten layer is frequently not uniform across the wafer. If the etching step clears the dielectric of tungsten on the thicker side, the tungsten in the windows will be severely etched on the thinner side. If the etching just clears the dielectric on the thinner side, the dielectric will not be cleared of tungsten on the thicker side. In neither case will the tungsten and the dielectric form a planar surface. Alternatively, tungsten may be deposited selectively in the windows. However, tungsten is almost invariably deposited, at least to a limited extent, on the dielectric as well. This is, of course, undesirable.
Planar surfaces are desirable because they facilitate subsequent processing. For example, one approach to the problem of electrically contacting the devices simplifies the problem by putting electrical leads on several levels rather than the one level more commonly used at present in integrated circuits. Such an approach simplifies the geometric problems involved in contacting all devices which arise with a single level metal approach. However, the multi-level approach requires the formation of electrical contacts between different metal levels. The problems are similar to those which arise in forming contacts to individual devices. Ultimately, of course, the hope is for circuits with metals on more than two levels as this may allow still more devices per unit area.